Hurricanes and Climate Change: Huge Dangers, Huge Unknowns

Hurricane Sandy's enormous $65 billion price tag put that great storm in third place for the most expensive weather-related disaster in U.S. (and world) history, and six of the ten most expensive U.S. weather-related disasters since 1980 have been hurricanes. Thus, how the strongest hurricanes may be affected due a changing climate is a topic of critical concern. Since hurricanes are heat engines that extract heat energy from the oceans to power themselves, hurricane scientists are confident that the very strongest storms will get stronger by the end of the century, when Earth's land and ocean temperatures are expected to warm 2 - 3°C, to levels unmatched since the Eemian Era, 115,000 years ago. Computer modeling work consistently indicates that greenhouse warming will cause the globally averaged intensity of tropical cyclones to shift towards stronger storms, with intensity increases of 2–11% by 2100. But hurricanes are fussy creations, and are sensitive to wind shear and dry air. Although the strongest storms should get stronger when "perfect storm" conditions are present, these "perfect storm" conditions may become less frequent in the future, due to the presence of higher wind shear, altered atmospheric circulation patterns, or more dry air at mid levels of the atmosphere. Indeed, the climate models used to formulate the 2007 IPCC report suggested that we might see the strongest hurricanes getting stronger, but a decrease in the total number of hurricanes in the Atlantic (and worldwide) later this century. However, the latest set of models used to formulate the 2013 IPCC report left open the possibility that we might see in increase in the total number of hurricanes, and and increase in their intensity. Given the conflicting model results, we really don't know how global warming will affect the number of hurricanes and their intensity, but we run the risk of making one of humanity's greatest scourges worse.

Figure 1. The list of most expensive U.S. weather-related disasters since 1980 is dominated by hurricanes.

Climate models and hurricane frequencyThe database we have on historical hurricanes does not extend far enough into the past and is not of high enough quality to make many judgements on how human-caused climate change may be affecting these great storms. A landmark 2010 review paper, "Tropical Cyclones and Climate Change", authored by ten top hurricane scientists concluded that the U.S. has not seen any long-term increase in landfalling tropical storms and hurricanes, and that "it remains uncertain whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes" (tropical cyclone is the generic term which encompasses tropical depressions, tropical storms, hurricanes, and typhoons.) Based in part on modeling studies using climate models run for the 2007 IPCC report, the scientists concluded that "it is likely that global mean tropical cyclone frequency will either decrease or remain unchanged owing to greenhouse warming." For example, one of the modeling studies the review paper quoted, Knutson et al. (2008), "Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions", projected a decrease in Atlantic tropical storms by 27% and hurricanes by 18% by the end of the century. An important reason that their model predicted these decreases was due to a predicted increase in wind shear. As I explain in my wind shear tutorial, a large change of wind speed with height over a hurricane creates a shearing force that tends to tear the storm apart. The amount of wind shear is critical in determining whether a hurricane can form or survive.

But a July 2013 study by MIT's Dr. Kerry Emanuel, "Downscaling CMIP5 climate models shows increased tropical cyclone activity over the 21st century", challenged this result. Dr. Emanuel argued that tropical cyclones are likely to become both stronger and more frequent as the climate continues to warm. This increase is most likely to occur in Western North Pacific, with smaller increases in the Atlantic. Dr. Emanuel took output from six newer higher-resolution climate models used to formulate the 2013 IPCC report, and used the output to drive a high-resolution hurricane model. The simulations found that the global frequency of tropical cyclones would increase by 11% to 40% by 2100, with intensity increases as well. The combined effects produced a global increase in Category 3 and stronger hurricanes of 40%. The behavior of these strongest hurricanes is critical, since they do most of the damage we observe. Over the past century, Category 3 - 5 hurricanes accounted for 85% of US hurricane damage, despite representing only 24% of U.S. landfalling storms. Category 4 and 5 hurricanes made up only 6% of all U.S. landfalls, but accounted for 48% of all U.S. damage (if normalized to account for increases in U.S. population and wealth, see Pielke et al., 2008.)

Figure 2. Projected changes in tropical cyclone track density during the 2006-2100 period compared to the 1950-2005 period, using output from six climate models included in the 2013 IPCC report. The global frequency of tropical cyclones is predicted to increase by 11% to 40%, with the largest changes occurring in the Northwest Pacific off the coast of Japan. Smaller increases are predicted for the Atlantic and near Australia. Image credit: Kerry Emanuel, "Downscaling CMIP5 climate models shows increased tropical cyclone activity over the 21st century", Proceedings of the National Academy of Sciences, July 8, 2013, doi: 10.1073/pnas.1301293110.

However, a study by Knutson et al. (2013), using the same latest-generation climate models as used by Emanuel (2013), but using the output from the models to drive a different high-resolution hurricane model, found a 20% decrease in Atlantic tropical storms and hurricanes by 2100. Two other 2013 studies by Villarini et al. and Camargo, also using output from the 2013 IPCC models, found essentially no change in Atlantic tropical cyclones. The reason for the differences, lies, in part, with how much global warming is assumed in the studies. Dr. Emanuel's study, which found an increase in tropical cyclone activity, assumed a worst-case warming situation (RCP 8.5), following the "business as usual" emissions path humanity is currently on. The Knutson et al. study, which found a decrease of 20% in Atlantic tropical cyclones, used a scenario (RCP 4.5) where it was assumed humans will wise up and cause about half of the worst-case greenhouse warming. The study found found "marginally significant" increases in Atlantic Category 4 and 5 hurricanes of 39% - 45% by 2100. These dramatically different results give credence to Dr. Emanuel statement at the end of his paper, "the response of tropical cyclones to projected climate change will remain uncertain for some time to come." The 2013 IPCC report also emphasized the high amount of uncertainty in how climate change might affect hurricanes, stating that there was "low confidence" that we have observed any increases in intense tropical cyclones due to human causes. However, since the 1970s, it is virtually certain (99 - 100% chance) that the frequency and intensity of hurricanes and tropical storms in the North Atlantic has increased, and there is medium confidence that a reduction in small air pollution particles (aerosols) over the North Atlantic caused part of this effect. The report's forecast for the future stated that it is "more likely than not" (50 - 100% chance) that human-caused climate change will cause a substantial increase in intense tropical cyclones in some ocean basins by 2100, with the Western North Pacific and Atlantic being at particular risk. Also, there will likely (66 - 100% chance) be an increase in both global mean tropical cyclone maximum wind speed and rain rates by 2100, and more likely than not (50 - 100% chance) that the increase in the most intense tropical cyclones will be larger than 10% in some basins.

CommentaryHurricane damages are currently doubling every ten years without the effect of climate change, according to Pielke et al., 2008. This is primarily due to the increasing population along the coast and increased wealth of the population. The authors theorize that the Great Miami Hurricane of 1926, a Category 4 monster that made a direct hit on Miami Beach, would have caused about $150 billion in damage had it hit in 2005. Thus, by 2015, the same hurricane would do $300 billion in damage, and $600 billion by 2025. This is without considering the impact that accelerating sea level rise will have on storm surge damages. Global sea level rise over the past decade has been about double what it was in the 20th century, and the rate of sea level rise is expected to increase further in the coming decades. Storm surge does the majority of damage in major hurricanes, and storm surges riding on top of higher sea levels are going to do a lot more damage in the coming decades. If we toss in the (controversial) increases in Category 3 and stronger storms like Dr. Emanuel suggests may occur, the hurricane damage math gets very impressive. We can also add onto that the relatively non-controversial increase in tropical cyclone rainfall of 20% expected by 2100, which will sharply increase damages due to fresh water river flooding. It is controversial whether or not we are already be seeing an increase in heavy precipitation events associated with tropical cyclones in the U.S., though. The total number of daily rainfall events exceeding 2" associated with tropical cyclones in the Southeast U.S. on a century time scale has not changed significantly, according to Groisman et al., 2004. But a 2010 study by Kunkel et al., "Recent increases in U.S. heavy precipitation associated with tropical cyclones", found that the number of Southeast U.S. tropical cyclone heavy precipitation events, defined as 1-in-5-year events, more than doubled between 1994 - 2008, compared to the long-term average from 1895 - 2008.

Figure 4. Time series of the 15-year running average (plotted at the end point of the 15-yr blocks) of a Tropical Cyclone Heavy Precipitation Index (red) and 15-year running average of U.S. landfalling hurricanes (blue). Note that there has been no long-term increase in U.S. landfalling hurricanes, but there has been a sharp increase in extreme rainfall events associated with landfalling tropical cyclones--the kind of rainfall events most likely to cause damaging flooding. Image credit: Kunkel et al. (2010), "Recent increases in U.S. heavy precipitation associated with tropical cyclones", Geophysical Research Letters.

It is essential that we limit coastal development in vulnerable coastal areas, particularly along barrier islands, to reduce some of the astronomical price tags hurricanes are going to be causing in the future. Adoption and enforcement of strict building standards is also a must, as well as more reforms to the government's National Flood Insurance Program (NFIP), which subsidizes development in high-risk coastal regions that private insurers won't touch. NFIP is now $25 - 30 billion in the red, thanks to Hurricane Katrina and Hurricane Sandy. Reform of NFIP is already underway. In 2012, before Sandy hit, Congress passed the Biggert-Waters Flood Insurance Reform Act, which requires people with NFIP policies to pay large premium increases of about 25% per year over the next five years. Naturally, this move has caused major controversy.

What the official climate assessments say about climate change and hurricanesThe 2013 IPCC report gives “low confidence”--a 20% chance--that we have observed a human-caused increase in intense hurricanes in some parts of the world. This is a reduction in odds from the 2007 report, which said that it was more likely than not (greater than 50% chance.) The IPCC likely took note of a landmark 2010 review paper, "Tropical Cyclones and Climate Change", authored by ten top hurricane scientists, which concluded that the U.S. had not seen any long-term increase in landfalling tropical storms and hurricanes, and that "it remains uncertain whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes." The 2013 IPCC report predicts that there is a greater than 50% chance (more likely than not) that we will see a human-caused increase in intense hurricanes by 2100 in some regions; this is also a reduction from the 2007 report, which said this would be likely (66% chance or higher.)

The May 2014 United States National Climate Assessment found that “The intensity, frequency, and duration of North Atlantic hurricanes, as well as the frequency of the strongest (Category 4 and 5) hurricanes, have all increased since the early 1980s. The relative contributions of human and natural causes to these increases are still uncertain. Hurricane-associated storm intensity and rainfall rates are projected to increase as the climate continues to warm.”

If it kept raining at that rate for a long time there would be a lot of local flooding. But luckily the rain rate has now dropped to 2.27" per hour and hopefully the T- Storm will move out of the area soon.

RECENT VISIBLE SATELLITE IMAGES SUGGEST THAT THE ORGANIZATIONOF THE SMALL HURRICANE HAS CONTINUED TO IMPROVE WITH A SMALLEYE BECOMING EVIDENT. A SERIES OF MICROWAVE IMAGES DURING THEPAST 12 HOURS OR SO HAVE CLEARLY SHOWN A CONTRACTING EYE.THE INITIAL INTENSITY HAS BEEN INCREASED TO 80 KT...WHICH IS BETWEENTHE SUBJECTIVE AND OBJECTIVE DVORAK ESTIMATES OF 77 KTAND 85 KT...RESPECTIVELY. HENRIETTE IS EXPECTED TO REMAIN INAN ENVIRONMENT FAVORABLE FOR SOME ADDITIONAL INTENSIFICATIONDURING THE NEXT 12 TO 18 HOURS. AFTER THAT TIME...SLIGHTLYCOOLER SSTS AND A DRIER AND MORE STABLE AIR MASS SHOULD CAUSE AGRADUAL WEAKENING TREND TO BEGIN. THE NHC INTENSITY FORECASTIS A LITTLE ABOVE THE GUIDANCE DURING THE FIRST 12 TO 24 HOURS...THEN LIES BETWEEN THE INTENSITY CONSENSUS AND THE SHIPS MODELTHERAFTER.

HENRIETTE HAS BEEN MOVING ON A STEADY NORTHWESTWARD HEADING ATABOUT 9 KT TODAY. THE TRACK FORECAST REASONING REMAINS UNCHANGEDFROM THE PREVIOUS ADVISORY. HENRIETTE IS EXPECTED TO CONTINUENORTHWESTWARD FOR ANOTHER DAY OR SO...THEN TURN WESTWARD AS ARIDGE RE-BUILDS TO THE NORTH OF THE CYCLONE. THE TRACK GUIDANCE ISAGAIN IN FAIRLY GOOD AGREEMENT THROUGH 48 TO 72 HOURS...EXCEPT FORSOME FORWARD SPEED DIFFERENCES. THE NHC TRACK DURING THIS TIMESPLITS THE DIFFERENCE BETWEEN THE SLOWER GFS AND THE FASTER ECMWFAND LIES NEAR THE MODEL CONSENSUS. AS HENRIETTE WEAKENS ANDBECOMES A MORE SHALLOW CYCLONE LATER IN THE FORECAST PERIOD...IT ISLIKELY TO TURN WEST-SOUTHWESTWARD IN THE LOW-LEVEL TRADEWIND FLOW. THE TRACK GUIDANCE AT DAYS 4 AND 5 SHIFTED SOUTHWARD THIS CYCLE ANDTHE OFFICIAL FORECAST HAS BEEN ADJUSTED IN THAT DIRECTION...BUT ITREMAINS ALONG THE NORTHERN EDGE OF THE GUIDANCE ENVELOPE AT THOSETIMES.

Less than a month ago the Gulf was covered in convection with multiple ULL's from Texas to Florida. Now for the last couple of weeks the Gulf has cleared which has helped warm the waters. It's only a matter of time before something tries to spin up there.

Even with the multiple ULL's and blobs that bought so much rain to the SE, the majority of the Gulf was still free on convection. What was, and is, particularly noticeable is the lack of any activity off the Yucatan and in the BOC. When I was sailing, I avoided that area like the plague in summer, not just because of the risk of tropical storms, but because there was so much convection that it made things really unpleasant. Except for the heat this year, the strong trades would have made sailing from the BOC down to Belize and over to the Caymans and Jamaica a pleasure. This is the kind of sailing I did in the spring, and never would have dreamed trying in summer. It's hard to understand how unusual this situation is unless you sailed the Gulf and Caribbean for about 30 years, as I did.

* Duration... heat index values in excess of 105 degrees and ashigh as 109 degrees are expected late Wednesday morning and afternoonfor several hours across the area.

Precautionary/preparedness actions...

A heat advisory means that a period of hot temperatures isexpected. The combination of hot temperatures and high humiditywill combine to create a situation in which heat illnesses arepossible. Drink plenty of fluids... stay in an air-conditionedroom... stay out of the sun... and check up on relatives andneighbors.

* PRIMARY THREATS INCLUDE... A FEW TORNADOES POSSIBLE SEVERAL LARGE HAIL EVENTS LIKELY WITH A FEW VERY LARGE HAIL EVENTS TO 2 INCHES IN DIAMETER POSSIBLE SEVERAL DAMAGING WIND GUSTS TO 70 MPH LIKELY

THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 80 STATUTE MILES NORTH AND SOUTH OF A LINE FROM 45 MILES NORTH NORTHWEST OF ORTONVILLE MINNESOTA TO 60 MILES EAST NORTHEAST OF MINNEAPOLIS MINNESOTA. FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU4).

PRECAUTIONARY/PREPAREDNESS ACTIONS...

REMEMBER...A TORNADO WATCH MEANS CONDITIONS ARE FAVORABLE FOR TORNADOES AND SEVERE THUNDERSTORMS IN AND CLOSE TO THE WATCH AREA. PERSONS IN THESE AREAS SHOULD BE ON THE LOOKOUT FOR THREATENING WEATHER CONDITIONS AND LISTEN FOR LATER STATEMENTS AND POSSIBLE WARNINGS.

&&

DISCUSSION...AS A 50+ KT WESTERLY 500 MB JET STREAK CONTINUES TO NOSE EASTWARD ACROSS THE NORTHERN PLAINS/UPPER MISSISSIPPI VALLEY REGION LATE THIS AFTERNOON...DESTABILIZATION IS EXPECTED TO BECOME SUFFICIENT FOR SUPERCELL DEVELOPMENT IN THE WARM SECTOR OF A WEAK SURFACE WAVE. WITH SOME STRENGTHENING OF SOUTHERLY LOW-LEVEL FLOW FIELDS...A REMNANT EFFECTIVE WARM FRONTAL ZONE ACROSS CENTRAL MINNESOTA INTO THE MINNEAPOLIS AREA WILL PROVIDE A POTENTIAL FOCUS FOR TORNADIC DEVELOPMENT THROUGH EARLY EVENING. OTHERWISE...THE EVOLUTION OF AN ORGANIZED MESOSCALE CONVECTIVE SYSTEM MAY EVENTUALLY OCCUR NEAR THE LOW AND ALONG A DEVELOPING COLD FRONT TRAILING TO THE WEST...WHICH SHOULD PROPAGATE EAST SOUTHEASTWARD WITH THE RISK FOR SEVERE WINDS AND HAIL ACROSS THE UPPER MISSISSIPPI VALLEY BY THIS EVENING.

AVIATION...TORNADOES AND A FEW SEVERE THUNDERSTORMS WITH HAIL SURFACE AND ALOFT TO 2 INCHES. EXTREME TURBULENCE AND SURFACE WIND GUSTS TO 60 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 450. MEAN STORM MOTION VECTOR 29035.

...KERR

Been trying to find a chaser with live feed in the area, but can't find one as of yet.

Quoting 1814. ackee:I think if comes next week none of the reliable models still show no real development I think we will enD up with 12 name storm or less with most Developing close to home just my view

Hi ackee! There is plenty of time to get 14-16 named storms, especially because we already have 4 named storms.

Understood, but 1 degree Celsius is 1.8 degrees Fahrenheit, so the difference is trivial. My point was that it really doesn't matter about the SST's if there are no other focusing mechanisms. The water could be 95, but, if there's a lack of vertical instability, nothing will happen. The whole "rocket fuel" theory means nothing until and unless there are other factors in place to cause storms. Just as a non-tropical example, we've had temperatures in the mid-90's and dewpoints in the high 70's, even the low 80's, over the last four days in SE Alabama. It's the kind of conditions that the flapping of the proverbial butterfly wings should have been enough to set off thunderstorms. Instead. we had mostly clear skies and no convection at all. Today, it's 95, but the dewpoint is actually lower, at 70, yet there are thunderstorms starting to form. We have a weak warm front coming in from the west that's providing just enough lift and instability to get storms started, even though conditions are not as "ideal" as they've been if you only look at temperature and dewpoint. We need some kind of focusing mechanism in the Gulf as well if we are to get storm formation. If that happens, warmer SST's then play a role. Until then, it just means people have a better time at the beach in nice warm water. :-)

* PRIMARY THREATS INCLUDE... A FEW TORNADOES POSSIBLE SEVERAL LARGE HAIL EVENTS LIKELY WITH A FEW VERY LARGE HAIL EVENTS TO 2 INCHES IN DIAMETER POSSIBLE SEVERAL DAMAGING WIND GUSTS TO 70 MPH LIKELY

THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 80 STATUTE MILES NORTH AND SOUTH OF A LINE FROM 45 MILES NORTH NORTHWEST OF ORTONVILLE MINNESOTA TO 60 MILES EAST NORTHEAST OF MINNEAPOLIS MINNESOTA. FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU4).

PRECAUTIONARY/PREPAREDNESS ACTIONS...

REMEMBER...A TORNADO WATCH MEANS CONDITIONS ARE FAVORABLE FOR TORNADOES AND SEVERE THUNDERSTORMS IN AND CLOSE TO THE WATCH AREA. PERSONS IN THESE AREAS SHOULD BE ON THE LOOKOUT FOR THREATENING WEATHER CONDITIONS AND LISTEN FOR LATER STATEMENTS AND POSSIBLE WARNINGS.

&&

DISCUSSION...AS A 50+ KT WESTERLY 500 MB JET STREAK CONTINUES TO NOSE EASTWARD ACROSS THE NORTHERN PLAINS/UPPER MISSISSIPPI VALLEY REGION LATE THIS AFTERNOON...DESTABILIZATION IS EXPECTED TO BECOME SUFFICIENT FOR SUPERCELL DEVELOPMENT IN THE WARM SECTOR OF A WEAK SURFACE WAVE. WITH SOME STRENGTHENING OF SOUTHERLY LOW-LEVEL FLOW FIELDS...A REMNANT EFFECTIVE WARM FRONTAL ZONE ACROSS CENTRAL MINNESOTA INTO THE MINNEAPOLIS AREA WILL PROVIDE A POTENTIAL FOCUS FOR TORNADIC DEVELOPMENT THROUGH EARLY EVENING. OTHERWISE...THE EVOLUTION OF AN ORGANIZED MESOSCALE CONVECTIVE SYSTEM MAY EVENTUALLY OCCUR NEAR THE LOW AND ALONG A DEVELOPING COLD FRONT TRAILING TO THE WEST...WHICH SHOULD PROPAGATE EAST SOUTHEASTWARD WITH THE RISK FOR SEVERE WINDS AND HAIL ACROSS THE UPPER MISSISSIPPI VALLEY BY THIS EVENING.

AVIATION...TORNADOES AND A FEW SEVERE THUNDERSTORMS WITH HAIL SURFACE AND ALOFT TO 2 INCHES. EXTREME TURBULENCE AND SURFACE WIND GUSTS TO 60 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 450. MEAN STORM MOTION VECTOR 29035.

Don't forget it only takes 1 storm. Hurricane Andrew (1st storm of the season) made landfall on Aug. 24th. Just imagine everyone sitting around in early August 1992 with 0 storms so far in the season saying this year's a bust.

That sat picture is about 12 hours before Andrew's center pasted 25 miles to the north of my house. 72 hours before no one thought we would get anything and if we did it would probably just be a TS, goes to show you things can change pretty quickly in the tropics.

Agree with that. By the way NOAA will release their August forecast on Thursday.

My issue with NOAA is that they seem afraid to put their neck on the block and give a precise forecast. Instead, they are ultra conservative and tell you for example to expect 13 - 20 named storms, 6 - 11 hurricanes and 2 - 6 major hurricanes.